Nanostructurally Controllable Strong Wood Aerogel toward Efficient Thermal Insulation

Eco-friendly materials with superior thermal insulation and mechanical properties are desirable for improved energy- and space-efficiency in buildings. Cellulose aerogels with structural anisotropy could fulfill these requirements, but complex processing and high energy demand are challenges for scaling up. Here we propose a scalable, nonadditive, top-down fabrication of strong anisotropic aerogels directly from wood with excellent, near isotropic thermal insulation functions. The aerogel was obtained through cell wall dissolution and controlled precipitation in lumen, using an ionic liquid (IL) mixture comprising DMSO and a guanidinium phosphorus-based IL [MTBD][MMP]. The wood aerogel shows a unique structure with lumen filled with nanofibrils network. In situ formation of a cellulosic nanofibril network in the lumen results in specific surface areas up to 280 m2/g and high yield strengths >1.2 MPa. The highly mesoporous structure (average pore diameter ∼20 nm) of freeze-dried wood aerogels leads to low thermal conductivities in both the radial (0.037 W/mK) and axial (0.057 W/mK) directions, showing great potential as scalable thermal insulators. This synthesis route is energy efficient with high nanostructural controllability. The unique nanostructure and rare combination of strength and thermal properties set the material apart from comparable bottom-up aerogels. This nonadditive synthesis approach is believed to contribute significantly toward large-scale design and structure control of biobased aerogels.


Supplementary figures
. Guanidinium-based and phosphorous-based ionic liquids studied   Figure S17. Pore-size distribution of freeze-dried 24h IL wood aerogel.  Tables   Table S1. Density, weight loss and dimensional changes of all samples Table S2. Thermal diffusivity of the tested samples 3. References 1. Chemistry

Materials and methods for ionic liquids preparation and characterization
Unless stated otherwise, all the chemicals and deuterated solvents were purchased from commercial sources at the highest degree of purity available and used without further purification. 1,1,3,3-Tetramethylguanidine (+98%) was purchased from Fluorochem and freshly distilled prior use. MTBD and MTBN were obtained from Liuotin group OY. Trimethylphosphate and Dimethyl methylphosphonate (DMMP) (+99%) were purchased from Merck or ABCR GmbH. The syntheses of ionic liquids were conducted under an argon atmosphere using conventional Schlenk techniques on a dual-manifold gas-inlet/vacuum line. All glassware was flame-dried prior to use. HPLC-quality grade reaction solvents were dried by conventional methods,. The solvents required to conduct the rest of synthetic operations were purchased at HPLC-quality grade (99.9% Honeywell, or Fischer Scientific) and used as received.
High-resolution electrospray-ionization mass spectra (ESI-MS) were recorded on a Bruker microTOF mass spectrometer operated in a positive or negative ion mode, using a 0.05 M solution of sodium formiate as a calibrant. Stock solutions (400-1000 ppm) were prepared of acetonitrile (99.9% Honeywell, Riedel-de Haën) and were diluted (1 ppm) with acetonitrile prior to the measurement. Only the prevalent ion peak (HCOO¯, H + or Na + adduct) is given for each compound.
IR was performed by Attenuated Total Reflect IR spectroscopy (ATR-IR) using an Alpha-P IR spectrometer from Bruker Optics. Thermogravimetric analysis (TGA) was used to determine the thermal stability of the ionic liquids on the range 30-500 °C using a Mettler Toledo TGA/DSC1, Switzerland device with a heating rate of 10 °C min −1 (N 2 atmosphere, 50 mL min −1 ). Differential scanning calorimetry (DSC) curves were obtained with a Mettler Toledo DSC1 instrument (Switzerland), using a heating rate of 10 °C min −1 between 30-500 °C under a nitrogen flow of 50 mL min −1 .

IL design
In contrast to conventional molecular liquids, ILs are amphiphilic in nature and behave like nano-heterogeneous media that self-assemble into polar and hydrophobic non-polar domains mainly governed by Coulombic electrostatic forces. S1,2 This amphiphilicity perfectly matches the anisotropic character of cellulose where relatively hydrophobic sheets, paired against one another, co-exist with polar chains linked by hydrogen bonding networks. S3 Therefore, ILs are endowed with an enormous potential to participate in key processes for aerogel formation such as the dissolution of lignocellulosic materials with unique control of the directionality of the molecular interactions involved. S3,4 To that end, the optimum ILs feature small anions with hydrogen-bond basicity high enough -such as halides or polyatomic anions with delocalized charge -to establish strong hydrogen bond interactions with the hydroxyls in equatorial positions of the cellulosic chains. S4,S5 Additionally, the ILs must contain small noncoordinating cations without long aliphatic chains or protic groups that obstruct cellulose dissolution by creating instead hydrogen bonds with the ILs anions. As a result, ILs cations also contribute to dissolve cellulose by establishing van der Waals and dispersion forces with the surfaces above and below the polar glucose chains, that replace the stacking hydrophobic interactions holding together the sheets in crystalline cellulose. S3,S6

S5
To begin our studies, we devised that ILs obtained from trimethyl phosphate (TMP) 4 and dimethyl methylphosphonate (DMMP) 5 could be ideal model solvents to evaluate the preparation of aerogels from wood materials ( Figure S1). S7,8 Indeed, such ILs are formed by methyl transfer from those phosphorous-based reagents, and thus can be protic or aprotic depending on the choice of cationic precursor accepting that methyl group. This versatility is key to access and evaluate ILs featuring different degrees of polarity, charge delocalization, and H-bonding interactions between their cation and anion constituents, all of which may be critical factors affecting wood dissolution and aerogel formation. S3 Furthermore, the resulting demethylated anions [DMP]and [MMP] -, in those ILs, possess hydrogen-bond basicity high enough to enable swift dissolution of relatively high loadings of cellulosic materials upon mild conditions. S9-10 However, to the best of our knowledge, imidazolium scaffolds are the only cations that have been hitherto used to prepare ILs featuring [DMP]and [MMP]anions. S7-10 To that end, the reported toxicity profile, S11,12 relative sensitivity to moisture and tendence to react with the cellulose reducing ends of imidazolium ILs, S13-15 prompted us to deviate our attention towards another family of N-based cations with delocalized charge such as guanidinium superbases 1-3, that has shown intriguing applicability in the processing of wood materials. S16-19 In practice, the preparation of the ILs with [DMP]anions, 6 and 9, was accomplished quantitatively by mixing the corresponding precursors in an equimolar ratio at 100-110 C. Conversely, the only IL-based on the [MMP]anion that could be obtained in such manner was [TMG] + [MMP] -7. In fact, the ILs containing bicyclic guanidinium cations and the [MMP]anion, 8 and 10, decomposed steadily over time when they were prepared by reacting equimolar amounts of their precursors. However, we were pleased to fully stabilize both ILs, 8 and 10, by using and slight excess of the corresponding basic guanidine precursor, 2 and 3, (ca. 0.2 equiv) in their synthesis. 10 Figure S1. Guanidinium-based and phosphorous-based ionic liquids included in this study S6

Screening of phosphorous-based guanidine type IL
With the set of guanidinium phosphorous-based ILs 6-10 in hand, we proceeded to pre-examine their reactivity profiles towards cellulosic materials. For this task, we selected Enocell as model pulp substrate due to its low hemicellulose content. Additionally, we took into account that electrolyte solutions, arising from the combination of ILs and some polar solvents such as DMSO, are less viscous and thus diffuse better into cellulosic biopolymers, hence leading to significant rate increases of cellulose dissolution. S20,21 Based on our previous work, S22 we selected IL:DMSO 80:20 (wt %) as the optimum ratio. This ratio has shown to be a reasonable compromise of dissolution at relatively low viscosity. This treatment with IL:DMSO 20:80 (wt %) electrolyte led to fibrillation or in some cases partial dissolution ( Figure S2).

IL electrolyte experiments:
In first place IL electrolyte solution was prepared by mixing 800 mg of DMSO-d 6 and 200 mg of the selected IL. In second place, a 4 ml vial was load with 0.03 g of Enocell pulp and 1 g of electrolyte solution (3 wt. % pulp consistency). The mixture was heated to 65 °C overnight under vigorous stirring. An aliquot of these mixtures was used directly for NMR analyses and another aliquot was directly analyzed by light-polarized microscopy (microscope Olympus BX51) to study the IL effect on pulp dissolution and particle morphology.

Synthesis and characterization of [MTBD] + [MMP] -
Distilled MTBD (613 g, 3.6 mol, 1.2 equiv), previously stored under Argon, was poured in a 2 L twonecked round-bottom flask with dimethyl methylphosphonate (DMMP) (384 g, 3 mol, 1.0 equiv). Then, the flask was evacuated and backfilled with Argon. Subsequently, the mixture was allowed to stand at room temperature for 10 min to gently dissipate the mild exothermic reaction observed and warmed up at 110 °C with vigorous stirring. Once the reaction was completed (ca. 18 h, as it was observed by NMR analysis of a reaction aliquot), the mixture was allowed to cool down at room temperature. The titled ionic liquid was obtained as a highly pure dark yellow liquid that did not require further purification.  , 2913, 2879, 2823, 1605, 1564, 1507, 1448, 1399, 1321, 1282, 1225, 1177, 1076, 1046.   Figure S9. Wood morphology after 24 hours of IL treatment. Here networks are seen to fill every fibre of the wood in radial, transversal and longitudinal direction. Starting from the right hand-side, the remarkable homogeneity is accentuated showing filling of every lumen of the entire wood piece of about 4 mm. The second image portrays the filled fibers from a cut along the fiber direction. The last image shows an intercept of the radial and longitudinal direction. Figure S10. High magnification of lumen fibril network structure from 24h treated aerogels. S15 Figure S11. Scanning electron microscope images of cross-section cut a) 12h Delignified wood (DW), and IL treated DW for b) 3h, c) 6h, d) 12h, e) 24h and f) 48h. Figure S12. Visual dimensional changes along ionic liquid treatment. Figure S13. Upscaling of wood aerogels. Figure S14. Average carbohydrate content for native wood, delignified wood and all IL treatment times. Glucose monosugars represents cellulose, whereas arabinose, galactose, mannose and xylose are the main constituents in hemicelluloses. Figure S15. a) Transversal tensile strain of all specimens with standard deviation, b) typical stress-strain curves for each specimen Figure S16. Hot plate thermal conductivity measurements. A hot plate is sandwiched between two aerogels. When in operation, a metal plate is position under the screw to put some load on the samples, to improve the connection with the hot disk. Red cable on the left hand-side is for temperature measurements during the measurements. Figure S17. Pore-size distribution of freeze-dried 24h IL treated wood aerogel used for thermal insulation measurements. Figure S18. IR-images of each consecutive data point for native wood and wood aerogel in radial and axial direction of the wood. Time and direction are assign to each image in ascending order. The hot plate had a stable temperature of 70 °C. S20 Figure S19. Specific heat capacity of treated samples